Piezoelectric ceramic composition

ABSTRACT

Disclosed are piezoelectric ceramic compositions comprising, as the essential component, (Na 0 .5 Bi 0 .5)Bi 4  Ti 4  O 15 , or (Na 0 .5 Bi 0 .5) 1-x  M x  Bi 4  Ti 4  O 15  where M is at least one divalent metal element or (A1 0 .5 A2 0 .5) and A1 is at least one monovalent metal element, A2 is at least one trivalent metal element, and 0&lt;x≦0.5, and, as the side component, manganese in an amount of from about 0.7 to 3.0% by weight, relative to the essential component and in terms of MnCO 3 . The compositions have a piezoelectric constant, d 33 , of not smaller than 25 pC/N at temperatures above 500° C.

FIELD OF THE INVENTION

The present invention relates to piezoelectric ceramic compositions, inparticular, to those for piezoelectric ceramic devices to be used athigh temperature, i.e., above 500° C.

BACKGROUND OF THE INVENTION

Heretofore, piezoelectric ceramic compositions consisting essentially oflead titanate zirconate (Pb(Ti_(x) Zr_(1-x))O₃ with 0<x<1) or leadtitanate (PbTiO₃) have been widely used for piezoelectric ceramicdevices such as piezoelectric ceramic sensors. However, thosepiezoelectric ceramic compositions lose their piezoelectricity attemperatures above their Curie point, which is between 200 and 500° C.or so. Therefore, they could not be used as materials for piezoelectricceramic sensors acting at temperatures above 500° C.

Compared with lead titanate zirconate and lead titanate, the Curie pointof the layer-structured bismuth compound Na₀.5 Bi₄.5 Ti₄ O₁₅(hereinafter referred to as NBT) has a high value of about 670° C., andthere is a hope of NBT piezoelectric materials usable at hightemperatures. Other compositions derived from NBT by substituting a partof the moiety of NBT, (Na₀.5 Bi₀.5), with any of Ca, Sr, Ba, Pb, (K₀.5Bi₀.5) or the like are also known as piezoelectric materials having ahigh Curie point.

However, piezoelectric ceramic compositions consisting essentially ofsuch a layer-structured bismuth compound generally have a smallpiezoelectric constant, d₃₃, compared with those consisting essentiallyof lead titanate zirconate or lead titanate, and are thereforeunsatisfactory for piezoelectric ceramic sensors that act on the basisof d₃₃, such as accelerometer sensors, etc.

Techniques of adding manganese to NBT or NBT-derived compositionsproduced by substituting a part of the NBT moiety, (Na₀.5 Bi₀.5), withCa (hereinafter referred to as NBCT) have been disclosed in a report"Piezoelectricity in Ceramics of Ferroelectric Bismuth Compound withLayer Structure" (by S. Ikegami and I. Ueda, in Japanese Journal ofApplied Physics, 1974) and in a report of "Production of Grain-orientedCeramics, Their Characteristic, and Their Applications to ElectronicMaterials" (by T. Takenaka and K. Sakata, in Ceramics, published byCeramics Association of Japan, 1989). Ikegami et al. added manganese inthe form of MnO₂ to NBT in an amount of 5 mol % of MnO₂ (that is, in anamount of about 0.42% by weight in terms of MnCO₃). However, they saynothing about any improvement in the piezoelectric characteristics ofthe compositions produced. Takenaka et al. added manganese to NBT andNBCT in an amount of from about 0.1 to 0.2% by weight in terms of MnCO₃.However, the composition of NBT to which was added 0.1% by weight ofMnCO₃ has d₃₃ of 15.6 pC/N, and that of NBCT to which was added 0.2% byweight of MnCO₃ has d₃₃ of 20.8 pC/N. Thus, d₃₃ of the compositions ofTakenaka et al. is still lower than the 25 pC/N which piezoelectricceramic sensors require for practical application.

SUMMARY OF THE INVENTION

The object of the present invention is to provide piezoelectric ceramiccompositions having a piezoelectric constant, d₃₃, of not lower than 25pC/N at 500° C. or higher.

The piezoelectric ceramic composition of the first aspect of theinvention comprises an essential component of a formula:

    (Na.sub.0.5 Bi.sub.0.5)Bi.sub.4 Ti.sub.4 O.sub.15,

and a side component of manganese in an amount of from about 0.7 to 3.0%by weight relative to the essential component and in terms of MnCO₃.

The piezoelectric ceramic composition of the second aspect of theinvention comprises an essential component of a general formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x M.sub.x Bi.sub.4 Ti.sub.4 O.sub.15,

wherein M is at least one divalent metal element, and 0<x≦0.5, and aside component of manganese in an amount of from about 0.7 to 3.0% byweight relative to the essential component and in terms of MnCO₃.

These piezoelectric ceramic compositions of the invention have a Curiepoint of about 600° C., and therefore do not lose the piezoelectricityeven at high temperatures above 500° C. In addition, they have apiezoelectric constant, d₃₃, of not lower than 25 pC/N, and aretherefore well applicable to ceramic sensors and the like that require alarge piezoelectric constant.

In the piezoelectric ceramic composition of the second aspect of theinvention, M in the formula is preferably at least one of Ca, Sr, Ba andPb. This is the third aspect of the invention. As comprising a divalentmetal element of that type, the composition of the third aspect has alarger piezoelectric constant.

The piezoelectric ceramic composition of the fourth aspect of theinvention comprises an essential component of a general formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x (A1.sub.0.5 A2.sub.0.5).sub.x Bi.sub.4 Ti.sub.4 O.sub.15,

wherein A1 is at least one monovalent metal element, A2 is at least onetrivalent metal element, and 0<≦0.5, and a side component of manganesein an amount of from about 0.7 to 3.0% by weight relative to theessential component and in terms of MnCO₃. This piezoelectric ceramiccomposition of the invention has a Curie point of about 600° C., andtherefore does not lose the piezoelectricity even at high temperaturesabove 500° C. In addition, it has a piezoelectric constant, d₃₃, of notlower than 25 pC/N, and is therefore well applicable to ceramic sensorsand the like that require a large piezoelectric constant.

In the piezoelectric ceramic composition of the fourth aspect of theinvention, A1 in the formula is preferably at least one of K and Li, andA2 therein is preferably Bi. This is the fifth aspect of the invention.As comprising the combination of monovalent and trivalent metal elementsof that type, the composition of the fifth aspect a larger piezoelectricconstant.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The essential component of the piezoelectric ceramic compositions of thesecond through fourth aspects invention is NBT or an NBT derivative asformed by substituting a part of the NBT moiety, (Na₀.5 Bi₀.5), with adivalent metal element or with monovalent and trivalent metal elements,in order that the Curie point of the compositions much higher than 500°C.

The substitution of a part of the NBT, (Na₀.5 Bi₀.5), with such metalelements enlarges the piezoelectric constant of the compositionscomprising the NBT derivative. The partial substitution of the moiety(Na₀.5 Bi₀.5) must be so effected that the amount of the substituentelement is not larger than that of (Na0.5Bi₀.5). The type of the elementfor the substitution is not limited to one, but a plurality of differenttypes of elements may be used for the substitution.

The side component of the piezoelectric ceramic compositions of theinvention is Mn in order to make the compositions have a piezoelectricconstant, d₃₃, of not lower than 25 pC/N. Its content is from about 0.7to 3.0% by weight relative to the essential component and in terms ofMnCO₃ and preferably about 1 to 2% by weight.

Now, the invention is described in more detail hereinunder withreference to the following Examples, which, however, are not intended torestrict the scope of the invention.

EXAMPLE 1

First prepared were Na₂ CO₃, Bi₂ O₃, Tio₂ and MnCO₃ as the startingmaterials for piezoelectric ceramic compositions of the invention to beproduced herein. These materials were weighed to give compositions of acompositional formula:

    (Na.sub.0.5 Bi.sub.0.5)Bi.sub.4 Ti.sub.4 O.sub.15 +αMnCO.sub.3

where the unit α is % by weight, and mixed in wet in a ball mill forabout 4 hours to obtain mixtures. Each mixture was dried and thencalcined at 700 to 900° C. to obtain calcined products. Each calcinedproduct was roughly ground, to which was added a suitable amount of anorganic binder, wet-ground in a ball mill for 4 hours, and then sievedthrough a 40-mesh sieve to adjust the grain size of the resultingpowder. Thus were obtained binder-containing mixtures. Next, eachbinder-containing mixture was shaped under a pressure of 1000 kg/cm²into columnar discs having a diameter of 12.5 mm and a thickness of 5mm. These discs were baked in air to obtain columnar piezoelectricceramic discs. A silver paste was applied onto the both surfaces of eachdisc in the ordinary manner, and baked to form silver electrodes on thesurfaces. Then, these discs were polarized in an insulating oil at 150to 200° C. while imparting thereto a direct current voltage of from 5 to20 kV/mm for 10 to 30 minutes. Thus were obtained piezoelectric ceramicdevices.

The Curie point and the piezoelectric constant, d₃₃, of those devicesproduced herein were measured and are shown in Table 1, in which theasterisked samples are outside the scope of the invention.

                  TABLE 1                                                         ______________________________________                                                              Curie Point                                             Sample No.                                                                              α (wt. %)                                                                           (° C.)                                                                           d.sub.33 (pC/N)                               ______________________________________                                        *1        0.6         660       18                                            2         0.7         660       26                                            3         1.0         655       30                                            4         3.0         645       27                                            *5        4.0         635       Not                                                                           polarized                                     Control   0.0         665       15                                            Sample                                                                        ______________________________________                                    

As seen in Table 1, it is obvious that the samples in Example 1 have alarger piezoelectric constant, d₃₃, than the control sample with no Mnadded. In addition, it is confirmed that all the samples of theinvention have a Curie point of above 600° C. and are therefore usableunder high temperatures above 500° C.

The reasons why the amount of MnCO₃, α, are specifically defined hereinis mentioned below.

In the invention, α is defined to be 0.7≦α≦3.0. This is because if α issmaller than about 0.7% by weight as in sample No. 1, the piezoelectricconstant, d₃₃, of the composition is unfavorably smaller than 25 pC/N.

On the other hand, if α is larger than about 3.0% by weight as in sampleNo. 5, the piezoelectric ceramic of the composition could not bepolarized.

EXAMPLE 2

First prepared were Na₂ CO₃, Bi₂ O₃, TiO₂, CaO, SrO, BaO, PbO and MnCO₃as the starting materials for piezoelectric ceramic compositions of theinvention to be produced herein. These materials were weighed to givecompositions of a compositional formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x M.sub.x Bi.sub.4 Ti.sub.4 O.sub.15 +βMnCO.sub.3

where M is at least one of Ca, Sr, Ba and Pb, and the unit of β is % byweight, and mixed in wet in a ball mill for about 4 hours to obtainmixtures.

These mixtures were processed in the same manner as in Example 1 toproduce piezoelectric ceramic devices, for which the details are omittedherein.

The Curie point and the piezoelectric constant, d₃₃, of those devicesproduced herein were measured and are shown in Table 2, in which theasterisked samples are outside the scope of the invention.

                  TABLE 2                                                         ______________________________________                                        Sample                        Curie Point                                     No.     M      x     β (wt. %)                                                                         (° C.)                                                                         d.sub.33 (pC/N)                         ______________________________________                                        *11     Ca     0.1   0.6      670     20                                      12      Ca     0.1   0.7      670     29                                      13      Ca     0.1   1.0      665     35                                      14      Ca     0.1   3.0      655     29                                      *15     Ca     0.1   4.0      645     Not                                                                           polarized                               16      Ca     0.3   1.0      680     30                                      17      Ca     0.5   1.0      695     26                                      *18     Ca     0.6   1.0      700     16                                      19      Sr     0.1   1.0      635     32                                      20      Ba     0.1   1.0      620     37                                      21      Pb     0.1   1.0      645     38                                      Control Ca     0.1   0.0      670     18                                      Sample                                                                        ______________________________________                                    

As seen in Table 2, it is obvious that the samples of the invention inExample 2 have a larger piezoelectric constant, d₃₃, than the controlsample with no Mn added. In addition, it is confirmed that all thesamples containing any of Ca, Sr, Ba or Pb as M have a Curie point ofabove 600° C. and are therefore usable under high temperatures above500° C.

The reasons why the value x and the amount of MnCO₃, β, are specificallydefined herein are mentioned below.

In the invention, x is defined to be 0<x≦0.5. This is because, if x islarger than about 0.5 as in Sample No. 18, the piezoelectric constant,d₃₃, of the composition is unfavorably smaller than 25 pC/N.

β is defined to be 0.7≦β≦3.0. This is because if β is smaller than about0.7 as in Sample No. 11, the piezoelectric constant, d₃₃, of thecomposition is unfavorably smaller than 25 pC/N.

On the other hand, if β is larger than about 3.0 as in sample No. 15,the piezoelectric ceramic of the composition could not be polarized.

EXAMPLE 3

First prepared were Na₂ CO₃, Bi₂ O₃, TiO₂, K₂ CO₃, Li₂ CO₃ and MnCO₃ asthe starting materials for piezoelectric ceramic compositions of theinvention to be produced herein. These materials were weighed to givecompositions of a compositional formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x (A1.sub.0.5 A2.sub.0.5).sub.x Bi.sub.4 Ti.sub.4 O.sub.15 +γMnCO.sub.3

where A1 is at least one of K and Li, A2 is Bi, and the unit of γ is %by weight, and mixed wet in a ball mill for about 4 hours to obtainmixtures.

These mixtures were processed in the same manner as in Example 1 toproduce piezoelectric ceramic devices, for which the details are omittedherein.

The Curie point and the piezoelectric constant, d₃₃, of those devicesproduced herein were measured and are shown in Table 3, in which theasterisked samples are outside the scope of the invention.

                  TABLE 3                                                         ______________________________________                                        Sample                           Curie Point                                  No.    A1    A2     x    γ (wt. %)                                                                       (° C.)                                                                         d.sub.33 (pC/N)                      ______________________________________                                        *31    K     Bi     0.1  0.6     635     20                                   32     K     Bi     0.1  0.7     635     27                                   33     K     Bi     0.1  1.0     635     33                                   34     K     Bi     0.1  3.0     630     26                                   *35    K     Bi     0.1  4.0     620     Not                                                                           polarized                            36     K     Bi     0.3  1.0     620     29                                   37     K     Bi     0.5  1.0     605     26                                   *38    K     Bi     0.6  1.0     585     15                                   39     Li    Bi     0.1  1.0     635     31                                   Control                                                                              K     Bi     0.1  0.0     640     17                                   Sample                                                                        ______________________________________                                    

As shown in Table 3, it is obvious that the samples of the invention inExample 3 have a larger piezoelectric constant, d₃₃, than the controlsample with no Mn added. In addition, it is confirmed that all thesamples of the invention containing K or Li as A1 can be formed intopiezoelectric ceramic devices having a piezoelectric constant of notsmaller than 25 at 500° C. or higher.

The reasons why the value x and the amount of MnCO₃, γ, are specificallydefined herein are mentioned below.

In the invention, x is defined to be 0<x≦0.5. This is because if x islarger than about 0.5 as in Sample No. 38, the piezoelectric constant,d₃₃, of the composition is unfavorably smaller than 25 pC/N.

γ is defined to be 0.7≦γ≦3.0. This is because if γ is smaller than about0.7 as in Sample No. 31, the piezoelectric constant, d₃₃, of thecomposition is unfavorably smaller than 25 pC/N.

On the other hand, if γ is larger than about 3.0 as in sample No. 35,the piezoelectric ceramic of the composition could not be polarized.

Having been mentioned in detail hereinabove, the piezoelectric ceramiccompositions of the invention, which comprise, as the essentialcomponent having a Curie point of at least about 600° C., (Na₀.5Bi₀.5)Bi₄ Ti₄ O₁₅, or (Na₀.5 Bi₀.5)_(1-x) M_(x) Bi₄ Ti₄ O₁₅ (where M isat least one divalent metal element and 0<x≦0.5), or (Na₀.5 Bi₀.5)_(1-x)(A1₀.5 A2₀.5)_(x) Bi₄ Ti₄ O₁₅ (where A1 is at least one monovalent metalelement, A2 is at least one trivalent metal element and 0≦y≦0.5), and,as the side component, manganese in an amount of from about 0.7 to 3.0%by weight, relative to the essential component and in terms of MnCO₃,have a piezoelectric constant, d₃₃, of not smaller than 25 pC/N attemperatures above 500° C.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A piezoelectric ceramic composition comprising anessential component of general formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x M.sub.x Bi.sub.4 Ti.sub.4 O.sub.15,

wherein M is at least one divalent metal element or (A1₀.5 A2₀.5), A1 isat least one monovalent metal element, A2 is at least one trivalentmetal element and 0≦x≦0.5, and manganese in an amount of from about 0.7to 3.0% by weight relative to the essential component and in terms ofMnCO₃.
 2. A piezoelectric ceramic composition as claimed in claim 1, inwhich x is
 0. 3. A piezoelectric ceramic composition as claimed in claim2, in which the amount of manganese is from about 1 to 2% by weightrelative to the essential component and in terms of MnCO₃.
 4. Apiezoelectric ceramic composition as claimed in claim 1, in which x isgreater than
 0. 5. The piezoelectric ceramic composition as claimed inclaim 4, in which M is at least one divalent metal selected from thegroup consisting of Ca, Sr, Ba and Pb.
 6. The piezoelectric ceramiccomposition as claimed in claim 5, in which M is Ca.
 7. A piezoelectricceramic composition as claimed in claim 6, in which the amount ofmanganese is from about 1 to 2% by weight relative to the essentialcomponent and in terms of MnCO₃.
 8. A piezoelectric ceramic compositionas claimed in claim 5, in which the amount of manganese is from about 1to 2% by weight relative to the essential component and in terms ofMnCO₃.
 9. A piezoelectric ceramic composition as claimed in claim 4, inwhich the amount of manganese is from about 1 to 2% by weight relativeto the essential component and in terms of MnCO₃.
 10. A piezoelectricceramic composition as claimed in claim 4 in which M is (A1₀.5 A2₀.5)and is expressed by the general formula:

    (Na.sub.0.5 Bi.sub.0.5).sub.1-x (A1.sub.0.5 A2.sub.0.5).sub.x Bi.sub.4 Ti.sub.4 O.sub.15.


11. The piezoelectric ceramic composition as claimed in claim 10, inwhich A1 is at least one of K and Li, and A2 is Bi.
 12. Thepiezoelectric ceramic composition as claimed in claim 11, in which A1 isK.
 13. A piezoelectric ceramic composition as claimed in claim 12, inwhich the amount of manganese is from about 1 to 2% by weight relativeto the essential component and in terms of MnCO₃.
 14. A piezoelectricceramic composition as claimed in claim 11, in which the amount ofmanganese is from about 1 to 2% by weight relative to the essentialcomponent and in terms of MnCO₃.
 15. A piezoelectric ceramic compositionas claimed in claim 10, in which the amount of manganese is from about 1to 2% by weight relative to the essential component and in terms ofMnCO₃.
 16. A piezoelectric ceramic composition as claimed in claim 1having a Curie point above 600° C. and a d₃₃ of at least 26 pC/N.